This project will focus on the characterization of the nuclear InsP3R in live3r and comparison of its function with InsP3R of the endoplasmic reticulum (ER). Three isoforms of the InsP3R have been identified, two of which have been identified in hepatocytes. Ca2+ permeation appears similar among the InsP3R channel isoforms, but their regulation by Ca2+ and Insp3 differ. The cellular localization of each receptor isoform may be correlated with its functional properties. We hypothesize that the nuclear InsP3R differs from the reticular isoform in its regulation by Ca2+ and Insp3, and that additional differences are expected when comparing receptor modulation by other regulatory factors such as arachidonic acid, FKBP, and calcineurin. An understanding of these isoform-specific regulatory processes is necessary to explain differences between nuclear and cytosolic Ca2+ signaling in liver. The hypotheses to be tested include 1) Does the nucleus have a functionally distinct InsP3R? 2) Which cytoplasmic factors are functionally important in regulating the nuclear InsP3R? And 3) Which regions of the InsP3R are responsible for functional differences between the nuclear and reticular channels? The preliminary results presented here show for the first time that nuclear and reticular InsP3R are regulated by Ca2+ in an isoform-specific manner. The experiments outlined in this project will investigate the functional difference of nuclear and reticular InsP3R at the single channel level and will correlate the channel properties with cell and organ function. The results to be obtained will identify regulatory factors that determine isoform-specific Ca2+ signaling responses, how the cell regulates the channel isoforms to optimize cellular responses, and how this regulation goes awry in pathophysiological situations, such as liver fibrosis and regeneration. A long term goal will be to use the molecular information obtained in these studies to suggest useful treatments for individuals affected with liver disease.